1. Field of the Invention
The present invention relates to optical communications devices and, more specifically, to a device that compensates for different optical path lengths in an optical communications system.
2. Description of the Prior Art
In recent years, optical communication systems have become increasingly common. A simple optical communication system would include a first station and, at some distance away, a second station. An optical transmitter at the first station would be optically coupled to an optical receiver at the second station via an optical fiber. Likewise an optical transmitter at the second station would be optically coupled to an optical receiver at the first station. This would be a full duplex optical communications system. More complex systems include many transmitters connected to many receivers, over varying distances. In such systems, the connections between transmitters and receivers could be dynamic and effected by a matrix switch, capable of making many-to-many connections.
A typical receiver is designed to receive an optical signal having a signal strength within a predetermined range. The signal strength at the receiver is determined by the power of the transmitter and the optical path length of the optical fiber connecting the receiver to the transmitter. Each optical fiber causes a predetermined amount of attenuation per unit length of fiber. Therefore, if a long fiber is used for the connection, the natural attenuation of the optical fiber attenuates the transmitted signal so that the receiver receives a signal having a weaker signal strength than the original signal strength at the time of transmission. Receivers are generally designed to have a sensitivity so as to be able to receive an optical signal that is attenuated by a relatively long optical fiber.
When a relatively short optical fiber connects a transmitter to a receiver, an optical signal from the transmitter, being un-attenuated by a long optical fiber, could overwhelm or even damage the receiver. In such a case, an attenuator is added to the optical path between the transmitter and the receiver so that the optical signal transmitted therebetween is attenuated to the point where it has a signal strength within the range expected by the receiver.
Frequently, it is necessary to connect an optical analyzer to an optical communications system to analyze the transmission of data as it is being transferred by the system. An analyzer typically receives a signal, displays it, regenerates it to its originally transmitted signal strength, and then outputs the signal it to the receiver. If the optical path length between the transmitter and the analyzer is relatively short, then the technician must connect an attenuator to the input of the analyzer so that the signal received from the transmitter will not damage the analyzer. Likewise, if the optical path length from the analyzer to the receiver is relatively short, then the technician must connect an attenuator to the output of the analyzer, so that the regenerated signal from the analyzer will not damage the receiver.
Existing systems have the disadvantage of requiring the technician to connect attenuators when connecting an optical analyzer to an optical communications system. Existing systems also have the disadvantage of being susceptible damage when incorrect values of annenuation are employed.